Livinsky, I.V., and T.G. Zhukovska. “On Orders of Two Transformation Semigroups of the Boolean”. Carpathian Mathematical Publications 6, no. 2 (December 27, 2014): 317–319. Accessed April 28, 2024. https://scijournals.pnu.edu.ua/index.php/cmp/article/view/1362.